Judy Kaye in On the Twentieth Century. I was a huge Madeline Kahn fan, but she was not happy in the show and didn't seem to care who knew it. (I know others here saw Kahn when she was really "on", but I did not. The story didn't even make sense with Kahn sleepwalking through her part. To be fair, there was a video of John Cullum on line a few years ago talking about how he felt Kahn had been unfairly treated by the production, so there's another side to this story. I only know what I saw for myself.)
From Kaye's enthusiastic performance, I learned that it isn't enough to be supremely talented (as Miss Kahn was by all accounts), you have to act like you want to be there. The entire mock-opera style of the libretto was obscured until Kaye bounced out and played it to the hilt, I went back several times and now have seen Kaye in numerous other roles; she has never disappointed me.